Those login bots are persistent little devils, aren't they?
The bad news is that you can't mass delete the log entries from within Admin Tools. You would get a PHP timeout error, the purge would crash, possibly damaging your database. You can empty the entire table directly from the database through phpMyAdmin or a similar tool. This is not the recommended solution.
The good news is that there's an easier way. Go to Tools and Plugin Parameters. In the first setting, "Maximum security exceptions log entries" set the number of entries you want to keep in your log. Note that the log won't purge all at once, it will do it over time with each page load to avoid the PHP timeout.
Dale L. Brackin
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!
My time zone is EST (UTC -5) (click here to see my current time in Philadelphia, PA)